Capacitive detector for an electronic differential pressure transmitter

ABSTRACT

An electronic differential pressure transmitter for use with industrial processes to produce an output signal suitable for transmission to a remote station for operating, indicating, recording or control equipment. The transmitter includes a force bar to which an input force is applied, the resultant bar displacement being sensed by a capacitive detector coupled to an electronic circuit yielding a feedback signal proportional to the input force. The detector is formed by two spaced electrodes mounted on a dielectric plate, the gap therebetween being effectively varied by means of an armature movable relative to the electrodes.

United States Patent Levesque et al.

CAPACITIVE DETECTOR FOR AN ELECTRONIC DIFFERENTIAL PRESSURE TRANSMITTERAssignee:

Filed:

Inventors: Peter S. Levesque, Jenkintown; Max

Gaertner, Warminster, both of Pa.

Fischer & Porter Company,

Warminster, Pa.

Feb. 11, 1974 Appl. No.: 441,629

Related US. Application Data Division of Ser. No. 327,562, Jan. 29,1973, Pat. No.

US. Cl 317/249 R; ZOO/DIG. 1; 317/250 Int. Cl. HOIG 5/01 Field of Search317/249 R, 246, 250, 261,

317/258', ZOO/DIG. l; 333/24 C, 249 D References Cited UNITED STATESPATENTS Bleazey 317/249 D Duncan 317/250 Khouri... 317/261 Webb....317/249 R UX Borisov 317/249 R 1 Oct. 14, 1975 3,675,095 6/1971 Lehmann317/261 3,693,059 9/1972 Harris ZOO/DIG. 1 3,710,209 1/1973 Webb 317/246FOREIGN PATENTS OR APPLICATIONS 4,028 1909 United Kingdom 317/249 ROTHER PUBLICATIONS Birks; .l. B., Modem Dielectric Material, Heywood &Co., London, 1963, pp. 198 & 199.

Primary Examiner--E. A. Goldberg [57] ABSTRACT An electronicdifferential pressure transmitter for use with industrial processes toproduce an output signal suitable for transmission to a remote stationfor operating, indicating, recording or control equipment. Thetransmitter includes a force bar to which an input force is applied, theresultant bar displacement being sensed by a capacitive detector coupledto an electronic circuit yielding a feedback signal proportional to theinput force. The detector is formed by two spaced electrodes mounted ona dielectric plate, the gap therebetween being effectively varied bymeans of an armature movable relative to the electrodes.

1 Claim, 6 Drawing Figures CQOSS FL EIUIP5 CAPACITIVE DETECTOR FOR ANELECTRONIC DIFFERENTIAL PRESSURE TRANSMITTER RELATED APPLICATION Thisapplication is a division of the copending application Ser. No. 327,562filed Jan. 29, 1973, now US. Pat. No. 3,832,618 issued Aug. 27, 1974.

BACKGROUND OF THE INVENTION This invention relates generally toelectronic differential presure transmitters, and more particularly toan improved transmitter of the force balance type.

One important application of the force balance principle is in thedifferential pressure flow transmitter. In a transmitter of this type,an elongated force bar is pivoted about a transverse axis. An imputforce derived from a differential pressure capsule and corresponding tothe flow rate of the fluid to be measured is applied to the force bar toproduce a torque about its fulcrum. Also applied is a rebalance torquewhich tends to hold the bar motionless.

The rebalance torque is developed by a negative feedback loop thatincludes a detector to sense any slight change in force bar position dueto an unbalance of torques. The detector directs a correspondingfeedback signal to a feedback motor that applies to the force bar aforce in opposition to the input force. This feedback signal ismaintained proportional to the flow rate being measured and is usedtherefore to produce an output signal for transmission to a remotecontrol station or to an indicating or recording device.

In a force-balance instrument of the electrical type, the feedbacksystem is provided with an electric motor and the output signal iselectrical in nature, whereas in the pneumatic type, such as thatdisclosed in the copending application Ser. No. 212,585 of Siegel(common assignee), the motor is in the form of a pneumatically actuatedbellows and the output signal which is applied to the bellows is fluidicin nature.

In a force-balance transmitter, there is virtually no movement of theforce bar over the full-scale range of operation. This virtual absenceof movement is highly advantageous, for it effectively eliminateshysteresis and other errors of the type encountered in so-calledmotion-balance instruments. On the other hand, existing instrumentssuffer from certain drawbacks which have somewhat limited theirapplicability. For example, prior instruments are excessively sensitiveto positional orientation and vibration. Also, with existinginstruments, friction at contact points gives rise to inaccuracies.

In a force-balance instrument, it is important to be able to change theoperating range or span of the instrument. In an instrument of the typedisclosed in US. Pat. No. 3,564,923, in which the feedback system iselectrical in nature and employs a feedback motor, the force balancemechanism includes a vernier rangechanging structure in the form of aflexured reaction member and a rotatable support element adapted tochange range without altering the static balance of the instrument. Inthis patent, in order to effect relatively large change in range, thewinding of the motor is split into sets which are selectively connectedso as to alter the effective number of turns and thereby bring aboutgross step changes in the force developed by the motor armature.

LII

SUMMARY OF THE INVENTION In view of the foregoing, it is the main objectof this invention to provide a differential-pressure (D-P) transmitterof the force-balance type, which is of simplified and efficient design,the transmitter being adapted to generate and send a process-signalcurrent (normally 4 20 ma) that is proportional to the differentialpressure applied to the instrument.

More particularly, it is an object of this invention to provide a D-Ptransmitter of the above type, having a fixed mechanical span, the spanof the instrument being varied electronically by a current-splittingcircuit that shunts an adjustable percentage of the total current aroundthe force motor.

A significant feature of the invention is that the need for a spanadjustment mechanism is obviated, the mechanical advantage from theforce motor to the body having a high value and being fixed.

Also an object of the invention is to provide an electronic D-Ptransmitter in which displacement of the force beam is sensed by acapacitive detector whose value is varied as a function of beammovement, which detector is formed by two fixed electrodes in spacedrelation and an armature mechanically coupled to the force beam andarranged to vary the capacitance established between the fixedelectrodes.

An advantage of this detector structure lies in the fact that thearmature floats electrically, whereby changes in capacitance arepresented at the output terminals of the detector without the need formoving wires.

Yet another object of the invention is to provide an electronic circuitoperating in conjunction with a capacitive detector, and including afree-running multivibrator whose output duty cycle is indicative of thecapacitor value.

Briefly stated, these objects are attained in a differential-pressuretransmitter including a force bar to which an input force is applied,the resultant displacement of the bar being sensed by a detectorincluded in an electronic circuit to produce a feedback signal that isproportional to the input force and is fed to a motor that applies tothis force bar a rebalancing force in opposition to the input force.

Adjustment in span is effected by a span-adjust circuit in which thefeedback signal is split between the motor and a shunt path to a degreeproviding the desired span. The detector is preferably of the capacitivetype, and is associated with a free-running multivibrator whose outputduty cycle is indicative of the detector capacitance, the multivibratoroutput being integrated and amplified to produce said feedback signal.

OUTLINE OF THE DRAWING For a better understanding of the invention aswell as other objects and further features thereof, reference is made tothe following description to be read in conjunction with theaccompanying drawing, wherein:

FIG. 1 schematically shows the structure of a differential-pressuretransmitter in accordance with the invention;

FIG. 2 is a plan view of the capacitive detector included in thetransmitter and FIG. 2A is a plan view of an alternative configurationfor the electrodes of the capacitive detector;

FIG. 3 is a block diagram of the mechanism of the transmitter and theelectronic circuit associated therewith;

FIG. 4 is a schematic diagram of the electronic circuit; and

FIG. 5 is a simplified schematic of the zero-span adjust circuit.

DESCRIPTION OF THE INVENTION THE D-P TRANSMITTER STRUCTURE Referring nowto FIG. 1, there is illustrated in schematic form, adifferential-pressure transmitter in accordance with the invention. Thetwo major elements of the D-P transmitter are the measuring element thatsenses the existing differential pressure and converts it to a force,and the force-beam element that at one end receives the force from themeasuring element, and at the other end receives a rebalancing forcefrom a feedback motor. I

The measuring element is generally defined as that portion of theinstrument below the fulcrum of the force beam 10. This element includesa differentialpresure capsule provided with a pair of identical presurechambers 11 and 12 in which are mounted corrugated metal diaphragms 11Aand 12A. The diaphragms enclose a chamber containing a hydraulic fill13. Low and high fluidic pressure are applied to chambers 11 and 12,respectively.

In practice, suitable pipes carrying fluid under pressure are coupled tothe chambers, the pipes being connected upstream and downstream of anorifice plate inserted in a process line, whereby the differentialpressure between the upstream and downstream pipes is proportional tothe rate of fluid flow through the line. The D-P transmitter maytherefore be mounted directly in the line at a point remote from acentral control station to which the measurement data is sent in theform of an electrical signal. At the control station, the receivedsignal may be indicated or recorded, or used to operate process-controlapparatus.

In the instrument capsule, diaphragms l1 and 12 are joined together bymeans of a horizontal link 14 whose midpoint is connected to the lowerextremity of elongated force-beam which is pivotally supported in thevertical position by a seal diaphragm l5 and by vertical flexures (notshown). Because of the difference between the low and high pressurefluids applied to pressure-responsive diaphragms l1 and 12, the link 14tends to shift leftward to an extent depending on this difference,thereby applying a force to the lower ext'remity of beam 10.

The purpose of hydraulic fill 13 is to provide hydraulic damping fornoise, this being adjusted by throttling the flow or fluid in thepassage between the diaphragm sections by means of a needle valve 16inserted in the passage. Over range protection is provided by allowingthe diaphragm to seat in a nest in the motor body which has convolutionsmatching those of the diaphragm.

The force developed by the differential causes force beam 10 to swing ina clockwise direction about its fulcrum 17. Thus, produced in aforce-beam extension 18 secured to the upper end of beam 10 andprojecting upwardly therefrom, is a rightwardly directed force whichpulls on a connecting rod 19 attached at right angles to the upper endof extension 18. The other end of this rod is connected to across-flexure 20.

Also secured to cross-flexure 20 is fulcrum 21A of a motor arm 21, towhose lower portion is attached the coil 22 of a moving-coil orelectrodynamic type of force motor, generally designated by numeral 23.The coil is counterbalanced in order to reduce the effects of gravityand vibration. This is accomplished by means of a counterweight 24attached to the upper end of motor arm 21. The counterweight is providedwith balance-adjusting screws 25 and 26.

Damping of the motor is effected by a tubular aluminum bobbin 27 onwhich coil 22 is wound. The coil is movable axially with respect to thepole 28 of a permanent magnet 29, whose other end is anchored on theframe 30 of the motor. The motor is temperaturecompensated by shuntingmagnet 29 with a nickel-alloy shunt element 31 having a negativetemperature coefficient of permeability. While there is shown apreferred form of motor, the invention is also workable with known typesof feedback motors.

Also secured to cross-flexure 20 is the arm 32 of a capacitive detector,generally designated by numeral 33. The detector is constituted by adielectric plate 34 preferably made of borosilicate glass, whose face ismetallized in a pattern to define the fixed electrodes of the capacitor.I

As shown separately in FIG. 2, these fixed electrodes consist of acentral, disc-shaped active area 35 which is surrounded by a splitannular active area 36 spaced from the central area by a ring-shaped gap37. Electrode 35 is connected by a conductive strip to terminal 38, andelectrode 36 is connected by a conductive strip to terminal 39.Alternatively, the active areas may be constituted by a pair ofsemicircular areas 35' and 36 separated by a straight gap.

The capacitance established between the active areas 35 and 36 is variedby means of a metal armature which forms a variable air gap along theunderside of dielectric plate 34. The armature is mounted on theswingable detector arm 32 by means of a glass-ball swivel 40 whichincludes a set screw to hold the armature at an adjusted position. Thusthe armature 50 floats electrically, its physical position determiningthe capacitance of the electrodes which are connected by fixed wires tothe feedback circuit. There are no wires connected to moving parts ofthe capacitor, thereby avoiding failure as a result of repeated bendingof leads.

In the differential-pressure transmitter, detector 33, which isoperatively coupled to the force bar by detector arm 32, changes invalue as a function of force-bar displacement. The detector is includedin an electronic circuit, to be later disclosed, which generates afeedback signal that is applied to feedback motor 23. Feedback motor 23,through its motor arm 21, applies a force to the force bar throughconnecting rod 19 in opposition to the input force.

The mechanical span of the instrument, which is determined by themechanical advantage from the forcemotor to the meter body, is fixed,and the full advantage thereof is taken in a single pass. That is tosay, the motor force necessary to effect balance is developed by themotor arm 21 acting as a simple lever. In practice, a high mechanicaladvantage (24: l) is obtained by so positioning the pivot of the motorarm as to obtain a high degree of leverage.

TI-IE ELECTRONIC SYSTEM Referring now to FIG. 3, there is shown thefeedback circuit in block diagram form. It will be seen that an inputforce is applied to the forcebeam structure at the summing junction 41where the input force is compared with the feedback force produced bymotor 23. The resultant force difference acts on the effective springrate (gradient) of the transmitter mechanism to produce a displacementof the parallel-plate capacitive detector 33.

Detector 33 is connected to a free-running multivibrator 42 whose dutycycle is a function of the detector capacitance value. A free-running orastable multivibrator has no stable state (see Walston et al.,Transistor Circuit Design, pages 377-380). This change in duty cyclegives rise to a change in the differential average D-C voltage output ofthe multivibrator. This voltage change is integrated by integrator 43and applied to the output stage 44 for power amplification.

The current through output stage 44 is defined as total current. Thistotal current (4 to ma) is the signal output from the transmitter. Butnot all of the total current flows through force motor 23. Therelationship between total current and motor current is determined by aspan/zero adjust circuit 45. This relationship satisfies the followinglinear relation:

Motor Current S (Total Current) Z, wherein S is set by the spanadjustment, and Z is set by the zero adjustment.

The force generated by force motor 23 is proportional to the currentpassing through motor coil 22. This force is compared to the input forceat the forcesumming junction 41 to complete the closed loop. If theforce generated by the force motor is less than the input force, a forceunbalance is created. The detector 33 and integrator 43 are configuredto bring about an increase in motor current in response to thiscondition, thereby bringing the loop back into balance.

The function of integrator 43 is to provide a very high gain at zerofrequency (in order to drive the unbalance to zero) and to provide adecreasing gain and a constant gain band at higher frequencies in orderto achieve dynamic stability. This is referred to in the art as aproportional plus integral amplifier. (See Kuo- Automatic ControlSystems pages 141 143).

Voltage (7 v.) regulator 46 provides a constantvoltage supply for themultivibrator 42, the integrator 43, the zero/span adjust circuit 45,and a current regulator 47. The current regulator 47 provides a biascurrent (i.e., 0.7 mA) which includes the current required to operatethe amplifier A in the zero/span adjust circuit. The requirement forthis regulator is described in the following commentary on currentdistribution through the circuit.

The block diagram indicates the current path through the circuit.Current entering the power supply node' 48 is the total current. Thereare four branches leaving this node. These are:

1. Motor current 2. Shunt current 3. Amplifier current 4. Compensatingcurrent The current regulator performs the function of regulating thesum of the amplifier and compensating current at a fixed value D; so,

Total I Shunt I Motor I D The Zero/span adjust circuit is configured toset the relationship between Motor I and Shunt I. Since current D isregulated, this sets the relationship between Motor I and Total I, asrequired.

THE ELECTRONIC CIRCUITS We shall, in connection with FIG. 4, nowconsider the electronic circuits constituting the main elements of thesystem shown in block form in FIG. 3.

Multivibrator 42 is of conventional design, and includes cross-coupledtransistors Q and Q The collectors of transistors 0 and Q are connectedto the respective inputs of differential amplifier A in integrator 43.When, for example, the capacity of detector 33 which is included in themultivibrator circuit, is less than 22 pF- (with a gap of 0.0006 inchesbetween the detector plates, the detector capacity is 22pF), transistorQ will then be in its off state longer than transistor Q. As aconsequence, average voltage at the collector of transistor Q will begreater than the average voltage at the collector of transistor Q.

In other words, the collector of O is positive with respect to thecollector of Q when the gap of detector 33 exceeds 0.006 inches, and isnegative when the gap is less than 0.006 inches. When, however, the gapis exactly 0.006 inches, the voltage is zero.

Integrator 43 includes differential amplifier A whose output is fed to atransistor Q It can be shown that the transfer function of amplifier Ayields integral gain and proportional gain with roll-off at highfrequencies. The amplifier transfer function therefore includesproportional and integral terms. The integral term drives the positionerror to zero, and the proportional term helps stabilize the loop.

The capacitor C is required to stabilize the particular amplifierchosen, while resistor R is needed to set the current through theamplifier. Resistor R provides degeneration for the amplifier A whileresistor R limits the current output of transistor Q thereby limitingthe total current to a reasonable value under any condition ofoperation.

Output stage 44 includes a current amplifier transistor Q whose gain isdetermined by the ratio of resistor R in the base circuit and resistor Rin the emitter circuit. The 4 to 20 mA output appears at output terminalT. Resistor R in the collector circuit is required to establish theminimum current through the transmitter which must flow in order tostart the circuit when power is initially applied.

In practice, the system is powered by a regulated 7- volt supply.Voltage (7 v.) regulator 46 is a shunt regulator, and includestransistors Q and Q For any current in the operating range of theregulator, the voltage drop across the regulator is constant. Theregulated drop across the regulator is the drop across zener diode CRand transistor Q V Diode CR and transistor Q, are selected to have equaland opposite temperature coefficients to eliminate the effect oftemperature on the circuit.

We shall now analyze the circuit arrangement and operation of thezero/span adjust circuit 45 in which force motor 23 is connected inseries with diode CR A shunt path is provided by transistor Q, whoseemitter is connected to diode CR and whose collector is connectedthrough span adjusting potentiometer R to the motor. The adjustable armof span potentiometer R is connected to one end of a zero adjustingpotentiometer R whose adjustable arm is connected to one input of adifferential amplifier A Before considering the actual circuit of stage45, we shall, in connection with FIG. 5, consider a simplified versionof this circuit. In FIG. 5, it will be seen that the total current(Total I) is divided between motor 23 and the shunting transistor Q Thespan potentiometer R is divided into resistor sections A and B by itsadjustable arm. The differential amplifier A whose output is applied tothe base of shunting transistor Q cooperates therewith to hold theamplifier differential input to zero volts. It follows that:

Shunt I (A) Motor I (B) E,

where Shunt I is the current through transistor Motor I is the currentthrough motor 23, and E is the voltage applied to one input of thedifferential amplifier A Thus,

Motor I (B) E Shunt I But,

Motor I= Total I Shunt I Motor I (B) E Motor I Total I- A Motor I TotalI Motor I (B/A) (E/A) Resistors R and R shape the position versus spancharacteristic of the network so that it is approximately logarithmic.This yields a constant percentage of rate resolution over the range ofthe adjustment. The voltage drop across diode CR assures that somevoltage will be available to Amplifier A even when the motor current isvery small.

Thus the arrangement is such that the span of the transmitter is fixedmechanically, zero and span adjustment being effected entirely byelectronic means, potentiometer R serving for zero adjustment andpotentiometer R for span adjustment.

While there has been shown and described a preferred embodiment of anelectronic differential pressure transmitter in accordance with theinvention, it will be appreciated that many changes and modificationsmay be made therein without, however, departing from the essentialspirit thereof.

We claim:

1. A displacement-responsive capacitive detector comprising:

A. a dielectric plate constituted by borosilicate glass;

B. two spaced fixed flat electrodes mounted in a common plane on saidplate;

C. terminals connected to said electrodes; and

D. an electrically-floating planar metal armature parallel to the facesof said electrodes, an air gap existing between said armature and saidfaces, said armature being movable in response to a displacement forcerelative to the faces of said electrodes to effectively vary the air gaptherebetween, thereby changing the value of capacitance presented atsaid terminals as a function of displacement; said flat electrodes beingmetallized on one side of said plate and being constituted by a pair ofsemi-circular areas whose straight edges are separated by a straight airgap.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,912,9DATED October 14, 1975 'NVENTOR(S) 3 Peter S. Levesque and Max GaertnerIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 2, line 18 "the body" should have read the meter body Column 6,line 17 "0.0006" shoufi have read 0. 006

Sign ed arid Scaled this twentieth D y f Jan ary 1976 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Parentsand Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPATENT NO. I 3,912,988 DATED October 14, 1975 |NVENTOR(S) 3 Peter S.Levesque and Max Gaertner it is certified that error appears in theabove-identiiied patent and that said Letters Patent are herebycorrected as shown below:

Column 2, line 18 "the body" should have read the meter body Column 6,line 17 "0.0006" should have read 0.006

Signed and Ercaled this twentieth D y f Jan ry 1976 [SEAL] Arrest.-

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Parentsand Trademarks

1. A displacement-responsive capacitive detector comprising: A. adielectric plate constituted by borosilicate glass; B. two spaced fixedflat electrodes mounted in a common plane on said plate; C. terminalsconnected to said electrodes; and D. an electrically-floating planarmetal armature parallel to the faces of said electrodes, an air gapexisting between said armature and said faces, said armature beingmovable in response to a displacement force relative to the faces ofsaid electrodes to effectively vary the air gap therebetween, therebychanging the value of capacitance presented at said terminals as afunction of displacement; said flat electrodes being metallized on oneside of said plate and being constituted by a pair of semi-circularareas whose straight edges are separated by a straight air gap.